According to current regulations the safety measures to which gas delivery valve control boards, and more particularly the electrical control circuits which energise/de-energise valves through which combustible gas is delivered, are subjected are particularly stringent regulations. Among others these regulations apply to the boilers present for example in domestic heating systems.
In particular many “redundant” systems and devices to prevent the undesired delivery of gas if any component in the valve control circuit should fail or no longer function correctly must be provided within such circuits in order to comply with the reference regulations.
In general, in existing control circuits a microcontroller capable of controlling an actuator, for example a relay, to open/close a combustible gas delivery valve is often present. Because faults are also possible in the microcontroller itself, another control circuit must preferably “replace” the circuit included in the microcontroller if the latter should fail. In a possible embodiment this second control circuit may also include a supervisory element, such as a microcontroller, to control opening and closing of the valve through a separate signal delivered to the actuator (or to a separate actuator) so that the valve can open and deliver gas only in the situation where both the signals reach the actuator, which is then controlled in such a way as to permit the delivery of gas. If one of the two microcontrollers should fail, and if both should fail simultaneously, the valve will remain closed.
One of then disadvantages of this technical solution lies in the fact that because it is necessary to make these control circuits relatively economical so that they can be competitive in the market in question the presence of two microcontrollers results in an excessive increase in the final cost of the board controlling the valve.
British patent application GB 2229841 describes a fuel-heated device, for example a water heater, which has at least one fail-safe device which blocks delivery of fuel to the equipment's burner in the event of a fault, which is fed with electrical current and can only be deliberately unlocked through a control. In order to be able to maintain and use the fault information in this fail-safe device if there should be a power cut, the electronic fail-safe device is connected to a device which records the length of a power cut and which according to a preferred embodiment of the equipment comprises a non-volatile read-only semiconductor memory (EEPROM) which can be cancelled electrically.